1. Field of the Invention
This invention relates to Identification Friend or Foe (IFF) reply decoders for military and Air Traffic Control (ATC) service and more particularly to signal processing of Pulse Code Modulation (PCM) for detecting and declaring Mode 4 replies.
2. Description of the Prior Art
An Identification Friend or Foe (IFF) system includes an interrogator which transmits a challenge message in a particular sector and listens for replies from transponders in the sector the challenge message was transmitted in. Within the interrogator is a reply decoder which after some pre-processing must sort out all the returns from various decoder transponders and declare valid mode replies. The format or waveform of the mode 4 reply as used herein consists of three pulses which are spaced 1.75 microseconds apart. Thus when an interrogator transmits a mode 4 waveform the reply decoder is looking for three pulses which are spaced 1.75 microseconds apart. When three pulses spaced 1.75 microseconds apart and each pulse of a specified width are detected, a single pulse is provided to the reply decoder for processing to determine whether or not it qualifies as a valid mode 4 response. The single pulse, which is also a narrow pulse, represents a reply position referenced from the leading edge from the first pulse of the three pulse mode 4 waveform. This initial processing is performed in an associated leading edge detector. The decoding function for mode 4 waveforms is inherently complex, requiring correlation of data, by range, over several interrogation sweeps. Where the interrogation antenna is mechanically scanned or rotated, or electronically steered, target hits must be correlated over the azimuth of target illumination. Final processing of mode 4 target data is software controlled in the associated Reply Processor; but first, all varieties of False Replies Unsynchronized In Time (FRUIT) must be removed to the extent practical in the Reply Decoder to avoid overburdening the Reply Processor computer. The Reply Decoder must also validate replies from transponders to ensure acceptance of only valid replies from friendly transponders, which may be, for example, on an aircraft, ship, helicopter or ground based vehicle.
Under conditions of heavy reply rates, the probability becomes high that normal replies will be overlapped, interleaved, or closely spaced and in time alignment. Hardware means of deriving leading and trailing edges of reply pulses and then deducing the correct position of the original reply pulses will reduce these effects, but some such situations will still exist.
Considering the large number of interrogators in many localities, plus the typically large number of transponder-equipped aircraft that are within operational range of one or more of these interrogators at any give time, a large number of replies will be received by each interrogator station or platform. Only those valid responses to a particular interrogation are of interest to the respective interrogator; other replies, known as False Replies Unsynchronized In Time (FRUIT), cause major reply processing problems for the reply processor, and this problem becomes acute in high reply density areas.
Hardware means are included in reply decoders, and software means in reply processors, to minimize such FRUIT interference; but these means can not be substantially effective when "garbled" replies exist, i.e., where two or more replies arrive at the interrogator receiver at approximately the same time. The degarbling of overlapping replies is a major problem of all reply decoders. Obviously, it is desirable to eliminate reply garbling without losing valid replies from interrogated transponders.
Two additional problems are encountered with Mode 4 IFF operation that are not usually encountered with Air Traffic Control Radar Beacon System (ATCRBS), Selective Identification Function (SIF), and Traffic Alert and Collision Avoidance System (TCAS) applications: (1) Unfriendly platforms may intentionally generate interference or jamming signals; (2) Unfriendly platforms may also generate "spoofing" signals in an attempt to mimic the valid replies of friendly platforms. The crypto-secure Mode 4 format greatly diminishes changes of spoofing deception. Replies are controlled by a cryptographic computer, hereinafter referred to as "KIT Computer". Replies received by the interrogator receiver are passed through a second Cryptographic Computer, hereinafter referred to as KIR Computer, to aid reply decoding. The KIT computer is located with a transponder and the KIR computer is located with the interrogator.
Previous Mode 4 Reply Decoders have employed several means to filter out unwanted pulses. Hardware means are generally used to defruit the reply datastream; i.e., removal of pulses that are not synchronized with the interrogation. Correlation of replies obtained on successive interrogation sweeps allows several other means to reject unwanted responses or interference. Most interrogators use mechanically scanned or electronically steered antennas, so they interrogate a segment of azimuth at any one time. Friendly platforms with transponders will respond while they are illuminated by the interrogator's main beam of electromagnetic radiation. To evaluate a series of replies from a plurality of transponders, target start and stop algorithms are used where a record of hits and misses is maintained until target stop criteria is exceeded. Number of hits per interrogation and/or hits/unit time and/or hits in range gate/unit time can also be summed or totalized in hardware or software. Target azimuth width, azimuth center, and azimuth increments between hits can be derived for further target evaluation by the associated Reply Processor.
Background density against which friendly replies must be sorted out can also be measured for acceptance of correct friendly replies and rejection of enemy spoofs and unintentional friendly interference. For any given target range and interrogation sweep, there may be, for example, 15 incorrect reply range positions and only one correct position. The sum of incorrect replies present, excluding any reply that might be present at the one and only correct reply range position, is known as "density value".
If an unfriendly platform utilizes the simpler spoofing method of railing interference, it can be detected by looking for pulses that trail (in time) the current analysis reply position at intervals of 1.75 microseconds.
The KIR computer generates a challenge message which is encoded and transmitted to the transponder where it is decoded by the KIT computer which introduces a variable time delay prior to retransmitting a reply consisting of three pulses which are spaced 1.75 microseconds apart. In the prior art the received reply was first passed through the KIR computer which provided a complement time delay to the received reply so that the sum of the KIT computer delay and the KIR computer delay added up to a known fixed total time delay.